The present invention relates to enamelled wires having good self-lubricating properties.
Recently, manufacturers of electric apparatuses that use enamelled wires have come to use high-speed automatic winding machines to increase the speed of the production line. But during winding, the enamelled wire is subjected to friction and other mechanical stress and the insulation coating is damaged mechanically. If such damaged wire is assembled in an electric machine, layer shorting (short-circuiting between wires) occurs and the loss factor is increased to an undesirably high level. Therefore, to minimize the mechanical damage to the insulation coating, enamelled wires having good self-lubricating properties have been demanded. This demand should be met not only by the automatic winding machine but also when enamelled wires are inserted manually into a small slot in a motor. Since enamelled wires themselves do not have good self-lubricating properties, this demand has been met by coating the enamelled wire with a layer or various liquid lubricants such as liquid paraffin and refrigerator oil. The problem of vulnerability of enamelled wires to mechanical damage during winding has been partially solved by providing them with a nylon or polyamideimide overcoat having great mechanical strength or high scrape resistance.
However, to achieve further energy saving, additional improvement in the efficiency of various motors and transformers is desired, and this requirement is particularly great for motors immersed in refrigerants for coolers, air conditioners and refrigerators, and to meet this end, the space factor is increased by inserting more enamelled wires into the small slot in motors. For motors that are immersed in refrigerants, enamelled wires with a polyester imide or polyamideimide overcoat having not only high mechanical strength and scrape resistance but also high refrigerant resistance have conventionally been used. In addition, enamelled wires with a nylon overcoat have begun to be used in recent years. Damage to the insulation coating during winding has been prevented and the efficiency of inserting a coil of enamelled wires into the slot has been increased solely by coating the enamelled wires with refrigerator oil. But as more enamelled wires are inserted into the small slot to increase the space factor, hence the efficiency of motors, many problems have arisen that can hardly be solved by the conventional enamelled wires coated with liquid lubricants such as refrigerator oil. For one thing, liquid lubricants such as refrigerator oil do not have very high self-lubricating properties and slip properties, so a desired great number of enamelled wires cannot be inserted into the small slot, and if they are inserted with great force, the enamel coating is damaged mechanically to increase the chance of layer shorting. The low self-lubricating properties of the enamelled wire causes another disadvantage in that even after the coil of enamelled wire is inserted in the motor, the enamel coating is subjected to mechanical damage due to electromagnetic vibration, and as a result, layer shorting occurs and the motor fails to perform its function. Improving the self-lubricating properties of enamelled wires by applying a large quantity of liquid lubricants is little effective, and on the contrary, more dirt collects on the enamelled wires and the bonding strength of adhesive tape used to fix the end of the coil is adversely affected.
Attempts have been made to eliminate these defects by coating enamelled wires with solid lubricants such as solid paraffin and carnauba wax having better lubricating properties and slip properties than liquid lubricants. But if the enamelled wires having a coating of these solid lubricants are applied to motors immersed in refrigerants, the lubricant coating is extracted with the refrigerant and can clog the opening of the compressor valve or the refrigerant expansion nozzles in the refrigerator, to thereby reduce the refrigerating capacity of the machine. In addition, if the lubricant is extracted with the refrigerant, the self-lubricating properties and slip properties of the enamelled wire are reduced and the enamel coating becomes vulnerable to mechanical damage due to electromagnetic vibration. Furthermore, the solid lubricants are applied to the enamelled wire from a solution having a few percents of the lubricant dissolved in solvents such as petroleum benzine and xylene, but using a large quantity of low-boiling solvents is not only hazardous to human health but it also produces electric wires with creasing that do not have commercial value, and therefore, the coating of solid lubricants can only be applied to limited types of electric wires.
Another method that has been proposed for providing enamelled wires with high self-lubricating properties is to use enamel insulating paint compositions containing synthetic resins having good lubricating properties such as polyethylene, polypropylene and polytetrafluoroethylene, silicone oil, fluorine containing surfactants, and liquid as well as solid lubricants such as paraffin wax, carnauba wax and montan wax. But synthetic resins such as polyethylene, polypropylene and polytetrafluoroethylene are sparingly soluble in wire enamel compositions and are difficult to disperse in the wire enamel uniformly, and the resulting enamel is not highly stable. What is more, these resins are not highly miscible with the insulating components of the enamel so they are difficult to disperse in the insulation coating uniformly and the resulting enamel coating does not have good appearance. Liquid lubricants in the wire enamel composition provide an insulation coating whose slip properties and self-lubricating properties are as low as those of the coating formed by applying them onto the enamelled wire. Solid lubricants in the wire enamel composition are sometimes extracted with refrigerants or solvents after the wire enamel is applied to the electric wire (the same thing happens when solid lubricants are directly applied to the enamelled wire), and the enamelled wire so produced is difficult to apply to motors that are used in refrigerants. Furthermore, like synthetic resins, the solid lubricants are sparingly soluble in solvents for making wire enamel and they are not highly miscible with the insulating components of the enamel. Therefore, the resulting wire enamel is not stable and the lubricants are difficult to disperse in the insulation coating uniformly and hence the so produced coating does not have good appearance.